Barge Mounted Floating Power Plant

ABSTRACT

According to an embodiment of the disclosure, a vessel includes a hull structure, a foundation, a plurality of springs, and plurality of hydraulic jacks. The hull structure is configured to float on water. The foundation is mounted on top of the hull structure and has significant mass. The foundation is configured to support a turbine structure and to absorb at least a portion of the forces or kinetic energy from the turbine structure. The plurality of springs are positioned between the hull structure and the foundation. The plurality of springs are configured to isolate the foundation from the effects of deflections in the hull structure, to absorb at least a portion of the forces or kinetic energy transferred from the turbine structure, and to transfer at least another portion of the forces or kinetic energy from the turbine structure to the hull structure. The plurality of hydraulic jacks are positioned between the hull structure and the foundation and are configured to adjust the degree of stiffness between the hull structure and the foundation, and to absorb forces or kinetic energy from the turbine structure.

BACKGROUND

1. Field of the Invention

This invention is related generally to power plants and moreparticularly to a system and method for mounting an industrial gasturbine or generator on a vessel.

2. Description of Related Art

Large structures such as industrial gas turbines are conventionallypositioned on land. The mounting of such large structures on marinevessels has previously believed to have been impossible.

A principle object of the present invention is to provide a new systemand method for barge mounting an industrial gas turbine or generator,thereby overcoming previously known limitations. Embodiments of thepresent invention achieve these and other objectives provided hereinbelow.

SUMMARY

According to an embodiment of the present invention, a vessel includes ahull structure, a foundation, a plurality of springs, and plurality ofhydraulic jacks. The hull structure is configured to float on water. Thefoundation is mounted on top of the hull structure and has significantmass. The foundation is configured to support a turbine structure and toabsorb at least a portion of the forces or kinetic energy from theturbine structure. The plurality of springs are positioned between thehull structure and the foundation. The plurality of springs areconfigured to isolate the foundation from the effects of deflections inthe hull structure, to absorb at least a portion of the forces orkinetic energy transferred from the turbine structure, and to transferat least another portion of the forces or kinetic energy from theturbine structure to the hull structure. A plurality of hydraulic jacksare positioned between the hull structure and the foundation and areconfigured to adjust the degree of stiffness between the hull structureand the foundation, and to absorb forces or kinetic energy from theturbine structure.

Certain embodiments of the present invention may have a number oftechnical advantages. For example, some embodiments may allow themounting of an industrial gas turbine on a vessel. Some otherembodiments may absorb forces through one or more of hydraulic jacks,isolation springs, and a cement foundation. Some other embodiments mayadjust a degree of stiffness between a cement foundation and hullstructure of the vessel. Various embodiments may include some, all, ornone of the above advantages. Particular embodiments may include otheradvantages.

Before undertaking the DESCRIPTION OF EXAMPLE EMBODIMENTS below, it maybe advantageous to set forth definitions of certain words and phrasesused throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 is an elevation view of a floating structure with a mountingsystem, according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view of FIG. 1, cut across lines 2-2;

FIG. 3 is an elevation view of a floating structure with a mountingsystem, according to another embodiment of the present invention;

FIG. 4 is a cross-sectional view of FIG. 3, cut across lines 4-4;

FIG. 5A is an elevation view of a mounting system, according to anembodiment of the present invention;

FIG. 5B shows a zoomed-in portion of hydraulic jacks of FIG. 5A; and

FIGS. 6A and 6B show various forces acting on a hull structure,according to an embodiment of the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIGS. 1 through 7B, discussed below, and the various embodiments used todescribe the principles of the present invention in this patentapplication are by way of illustration only and should not be construedin any way to limit the scope of the disclosure. Those skilled in theart will understand that the principles of the present disclosure may beimplemented in any suitably arranged configuration. The invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, the disclosedembodiments are provided such that this disclosure will be thorough andcomplete, and will convey the scope of the invention to those skilled inthe art. The principles and features of the invention may be employed invaried and numerous embodiments without departing from the scope of theinvention.

This disclosure relates to the mounting of large industrial gas turbineson floating barges or other floating marine vessels for the supply oflarge capacity electrical energy to centers of population underemergency conditions or long term use. Conventionally, large industrialgas turbines have not been installed or mounted on barges due to avariety of concerns with such mounting. Among the concerns with themounting of such large structures on barges include concerns over deadloads and live loads. As one example live load concern, vibrationamplitudes during run up and at operating speeds need to besignificantly limited. This is because vibration sensors associated withthe gas turbine will automatically shut down the gas turbine if designamplitudes are exceeded. As one example dead load concern, the sheerweight of the turbines and potential deflections in the structure fromenvironmental conditions could prohibitively prevent a foundation fromadequately supporting the turbine—again resulting in a shut down of theturbine from sensor detection of anomalies in operation.

Given such concerns, teachings of certain embodiments of the disclosurerecognize a floating structure capable of supporting large structuresthrough features that mitigate dead load and live load concerns. Incertain embodiments, such structures (e.g., industrial gas turbines)will not know whether they are on land or on a floating structure.

FIG. 1 is an elevation view of a floating structure 50 with a mountingsystem 100, according to an embodiment of the present invention. FIG. 2is a cross-sectional view of FIG. 1, cut across lines 2-2. In thisembodiment, the floating structure 50 includes a mounting system 100that supports a gas turbine 210 and associated generator 220. Otherstructural components that may be utilized in the generation ofelectricity have been omitted for brevity. Collectively, as describedherein, the gas turbine 210, the generator 220, and other associatedenergy generation components may be considered the “turbine structure.”

The mounting system 100 includes a foundation 110, a hull structure 120,and springs 130. The hull structure 120 is generally shown floating inwater 84 having a water line 80 that separates water 84 from air 82.Each of the foundation 110, the hull structure 120, and the springs 130absorb and/or transfer energy that is imparted thereon—whether it bevibrations from the gas turbine 210 and associated generator 220 or fromdeflections that may occur in the hull structure 120. Each will bedescribed in more detail below.

Among the items that may be considered in the design of a floating gasturbine power generation facility are the following: the barge, thebarge's steel structures, and the fact that the barge is floating inwater. The effects of buoyancy and the distribution of weight of thebarge (including the gas turbine generator that may be loaded thereon)can create bending moments, bending stresses, and shear stresses in thehull girder or hull structure. These stresses, in turn, can cause thegirder to bend either in a sagging or hogging mode. The degree ofdeflection of the hull girder is the product of the bending moment andthe stiffness of the hull structure. The greater the stiffness of thehull structure, the lower the amplitudes of deflection.

Given such design concerns, the hull structures (e.g., hull structure120) of a floating power plant may be designed to reduce the amplitudesof deflection to a minimum while providing a stiff supporting structurefor the foundation (e.g., foundation 110) supporting the turbine andgenerator. However, even considering the stiffness of the hull structureand the distribution of weight and ballast along the hull structure toreduce bending moments, other external factors may also act upon thehull structure. One such factor is the thermal differential between thetemperature of water 84 and the temperature of the ambient air 82. Thedifference between the two may create thermal stresses in the hull,which, in turn, may create vertical deflections, sag, or both.Additionally, in particular configurations, the sun may create thermalstresses that cause both vertical and horizontal deflections of the hullgirder of amplitude. This may be dependent upon the strength of thesun's rays and their angle, which may constantly change.

With reference to FIGS. 1 and 2, to alleviate the affects of deflectionin the hull structure 120 on the foundation 110, energy absorption andtransfer structures such as springs 130 may be installed at load pointson the hull structure 120 in embodiments of the present invention. Thesprings 130 may isolate the foundation 110 from the effects of anydeflections in the hull structure 120. The foundation 110 may thus notbe affected by the differential stresses in the hull structure 120caused by bending and the like. Such stresses imparted on the hullstructure may therefore not be imparted to the foundation 110.

Additionally, the springs 130 in certain configurations may also helpuncouple the static and dynamic behavior of the gas turbine210/generator 220. That is, vibration forces from the gas turbine210/generator 220 may be at least partially absorbed by the springs 130.For those forces that are not absorbed by the springs 130, the springs130 may act as a conduit to translate a portion of the forces to thehull structure 120. The hull structure 120, in turn, may be made ofmaterials that dampen and/or translate forces.

Although springs 130 are shown in this embodiment, other types of energyabsorption and transfer structures may be utilized in other embodiments.For example, there are a variety of energy absorbing materials availableon the market that are designed to absorb vibrations.

To support the heavy static loads of the gas turbine 210 and generator220 and the even greater loads and forces created by the gas turbine 210and generator 220 when in operation, the foundation 110 may be designedsuch that it has significant mass. This significant mass, itself, helpsdampen vibrations from one or both of the gas turbine 210 and generator220. In particular configurations, the foundation 110 (or plinth) may beconstructed of reinforced steel and concrete. In certain configurations,the mass of the foundation 110 may be 1700 metric tons. In otherconfigurations, the mass of the foundation 110 may be more than or lessthan 1700 metric tons. In certain configurations, the foundation 110 mayhave a particular shape and be made of varying materials. For example,in particular embodiments, portions of the foundation 110 formed of aconcrete composite may be hollowed out and filled with Barite (alsoknown as Barium Sulphate) to absorb vibrations. In other configurations,other types of material may make-up some or all of portions of thefoundation 110 to absorb vibrations. Examples of configurations offoundation 110 are provided below as embodiments of the presentinvention.

In particular configurations of the present invention, the foundation110 may be viewed as a large reinforced concrete mass, supported on aseries of isolation springs 130 having sufficient internal strength toabsorb loads and partially impart them into the hull structure 120. Incertain configurations, the large mass of the foundation 110—byitself—may be insufficient to assure compliance with design factors, forexample, the avoidance of resonance conditions during operation thatwould create vibration amplitudes leading to the turbine being shutdown. Certain manufactures such a General Electric impose limits onvertical and transverse vibrations; if such limits are exceeded, the gasturbine automatically shuts down. This includes the varying frequenciesof the turbine during run up to its operating rotation per minute (rpm)and run down when stopping the turbine.

The foundation 110 thus may be designed with capability to avoid orabsorb resonance frequencies at each of the following three modes ofoperation: run up, operating, and run down. Each may cause vibrationamplitudes that shut down the turbine. In particular configurations, thedesign may avoid resonance at 60 Hz, the operating frequency of certainturbine generators. In other embodiments, the configuration may avoidfrequencies other than 60 Hz.

As a recapitulation of certain features above, the foundation 110,itself, may absorb certain forces or kinetic energy that are created bythe structure thereon, namely the gas turbine 210 and associatedgenerator 220. Those forces or kinetic energy not absorbed by thefoundation 110 may be absorbed by the springs 130. And, those forces orkinetic energy that are not absorbed by the foundation 110 and springs130 may be absorbed by the hull structure 120, which may have featuresdesigned to absorb energy. As referenced above, the springs 130 may alsoisolate deformations in the hull structure 120 from impacting the gasturbine 210 and associated generator 220. Additional stabilizationfeatures may also be incorporated according to certain embodiments.

FIG. 3 is an elevation view of a floating structure 60 with a mountingsystem 300, according to another embodiment of the disclosure. FIG. 4 isa cross-sectional view of FIG. 3, cut across lines 4-4. The floatingstructure 60 of FIGS. 3 and 4 is shown with similar features to thefloating structure 50 of FIGS. 1 and 2. In particular, the floatingstructure 60 include a gas turbine 210/generator 220 (shown in dashedview in FIG. 4) on top of a mounting system 300 with a foundation 310, adeck 322 of a hull structure 320, and springs 330. The mounting system300 of FIGS. 3 and 4 also includes a particular shape for the foundation310 and hydraulic jacks 340—each of which is described in more detailbelow. Further, a pedestals 315 for supporting the gas turbine210/generator 220 are shown. The pedestals 315 may be made of similar ordifferent material than the foundation 310.

Because it may not be possible to avoid all frequencies imparted by theoperation of the gas turbine 210/generator 220, the foundation 310 maybe designed with additional capability to absorb such frequencies. Inparticular embodiments, this may be accomplished through the use ofparticular geometric shapes on the lower part of the foundation 310along with the internal mix of the concrete and steel reinforcement. Asseen in FIG. 4, the foundation 310 has the shape of half of a letter Iflipped on its side. From the angle seen in FIG. 4, the foundation 310has a thinner thickness portion 312. From this thinner thickness portion312, there are angled portions 314 that lead to feet 316 of thefoundation 310. As shown, the feet 316 are in communication with thehydraulic jacks 340 and springs 330. A shape such as the one shown inFIG. 4 optimizes the structural value of the mass of the foundation 310.That is, a mass is placed in areas of greater structural value forsupport, force translation and absorption and not placed in areas withless value for support, force translation and absorption. Although aparticular shape is shown, other shapes may be utilized. Some of suchshapes may be determined through a structural analysis, which may bedependent on the load and materials utilized.

In addition to the above, the actual material for the foundation (whichmay be a mixture of concrete or other materials) may be based on ananalysis that considers a strength of the materials, speed of the curerate for materials requiring curing, and damping capacity of thematerials. As referenced above, a variety of materials may be used inthe foundation 310, making the foundation a composite of such materials.Such materials include, but are not limited to, various types ofconcrete with various mixtures including barite and steel. The overallaffect of the foundation design may be to increase its internal dampingcapability such that it is sufficient to absorb the affect of resonancefrequencies.

To ensure that vibration amplitudes do not exceed the limits imposed byturbines manufacturers such as General Electric, certain configurationsmay include two additional features. First, large structural brackets324 may be included in the hull structure. These large structuralbrackets 324 may be designed to absorb any transient horizontalvibration created movement that is imparted thereon. The structuralbrackets 324 may utilize any appropriate structural design techniques.

Second, a series of hydraulic jacks 340 may be installed between thefoundation 310 and the deck 322 of the hull structure. The hydraulicjacks 340 may be installed in precise locations to permit a degree ofthe turbine 210/generator 220 forces to be absorbed by the mass of thehull structure 320 and the water that the hull structure 320 is floatingin. In particular configurations, the hydraulic jacks 340 may adjust thestiffness between the deck 322 and the foundation 310 by, for example,removing or adding the effects of the springs 330. Additionally, inparticular embodiments, the hydraulic jacks 340 may be designed suchthat vibration frequencies are also absorbed by the fluid in the jacks340 and their associated accumulators.

FIGS. 5A and 5B provide additional details of embodiments of thedisclosure. FIG. 5A is an elevation view of a mounting system 500,according to an embodiment of the disclosure. FIG. 5B shows a zoomed inportion of hydraulic jacks 540 of FIG. 5A. The mounting system 500 ofFIGS. 5A and 5B is shown with similar features to the mounting system300 of FIGS. 3 and 4. In particular, the mounting system 500 includes afoundation 510, a deck 522 of a hull structure 520, hydraulic jacks 540,and support structures 524.

With reference to FIG. 5A, energy paths (indicated by arrows 525) can beseen along the support structure 524. The water 84 immediatelysurrounding the hull structure provides an added mass 86, which providesan added dampening for forces that may have traversed through thesupport structure 510, through the springs or hydraulic jacks 540, andthrough the hull structure 520 for ultimate final dampening by the addedmass 86.

With reference to FIG. 5B, further details of operation of the hydraulicjack 540 can be seen. In this configuration, the hydraulic jack 540includes a base 542 coupled to the deck 522 and an extendable piston 544coupled to the support structure 510. In operation, the introduction ofhydraulic fluid into the base 542 causes the extendable piston 544 tomove outward, extending the distance between the deck 522 and thesupport structure 510. In particular configurations, the hydraulic jack540 is in fluid communication with an accumulator 560. In particularconfigurations, the accumulator 560 may supply pressure to one or morehydraulic jacks 540. This pressure may be manipulated using a needlecontrol valve 546. Thus, in other words, the hydraulic jack 540 may betuned through a control signal that opens or closes the need controlvalve 546.

Although a hydraulic jack 540 has been shown in this embodiment, otherembodiments may utilize other structural features. For example, inanother configuration, a plurality of power screws may increase ordecrease a spacing between the foundation 510 and the hull structure520—effectively manipulating the amount of compression on the springs(in embodiments utilizing such springs).

FIGS. 6A and 6B show various forces acting on a hull structure 620,according to an embodiment of the disclosure. In FIG. 6A, theconcentrated load of the turbine, generator, and foundation is indicatedby arrow 682. Buoyant forces (indicated by arrows 684) resulting fromdisplaced water counteract this concentrated load. A difference in anaverage temperature of air 82 and an average temperature in water 84 canresult in a sagging condition in the hull structure 620 with thedeflection indicated by the dotted lines 627. To counteract such adeflection, particular embodiments may include solid ballasts 670 asshown in FIG. 6B. The configuration of the solid ballasts 670 may bedependent on the particular geography in which the support system mayultimately be placed, for example, the average air temperature andaverage water temperature in such a geographical location. In particularembodiments, such solid ballasts 620 may yield zero hull deflection asindicated by arrow 629.

The following is a recapitulation of certain features of variousembodiments described above:

-   -   A solid ballast may be installed in the hull structure to        produce a level trim and to produce a neutral condition of zero        bending of the hull structure with water and air temperatures        that equate to the average air and water temperature conditions        at the operating site.    -   The hull structure may be installed with very heavy support        structures below the foundation for the turbine and generator,        allowing any flow of loads into the hull structure to gain added        mass from the surrounding water of the hull structure to        increase damping.    -   Springs may be installed between the deck of the hull structure        and the bottom of the foundation to isolate the foundation from        any deformation of the hull structure and to provide a conduit        for the flow of loads from the foundation into the hull        structure.    -   Hydraulic jacks may be installed between the deck of the hull        structure and the bottom of the foundation to tune the        foundation system during initial commissioning.

In designing the foundation, one or more of the following may beconsidered:

-   -   Structural support of the load of the gas turbine generator set        during operation, including the forces produced by a short        circuit event of the generator.    -   Support by multiple springs with their associated unsupported        spans    -   A dimension and mass of the foundation, similar to a land based        foundation design such that the gas turbine/generator        installation conforms exactly to original manufacturer's intent.    -   A foundation that has a high internal damping capability to        absorb vibrations emanating from the gas turbine/generator        during operation.    -   Permitting the foundation to be partially loaded by the tuning        jacks that will modify the natural frequency at the        foundation/hull structure interface to reduce resonance and add        damping from the surrounding water of the submerged part of the        hull.

Although structures and materials may have been described, the presentdisclosure may not be limited to these specifics, and others may besubstituted as it is well understood by those skilled in the art, andvarious steps may not necessarily be performed in the sequences shown.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

Also, techniques, systems, subsystems and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other products shown or discussed as directly coupled or communicatingwith each other may be coupled through some interface or device, suchthat the products may no longer be considered directly coupled to eachother but may still be indirectly coupled and in communication, whetherelectrically, mechanically, or otherwise with one another. Otherexamples of changes, substitutions, and alterations are ascertainable byone skilled in the art and could be made without departing from thespirit and scope disclosed herein.

What is claimed is:
 1. A system comprising: a hull structure configuredto float in water; a concrete foundation mounted on top of the hullstructure, the concrete foundation configured to: support a turbinestructure, and absorb at least a portion of forces or kinetic energytransferred from the turbine structure to the concrete foundation. 2.The system of claim 1, wherein the concrete foundation has a mass of atleast 1700 metric tons.
 3. The system of claim 1, further comprising: aplurality of energy absorption and transfer structures positionedbetween the hull structure and the concrete foundation, the plurality ofenergy absorption and transfer structures configured to isolate theconcrete foundation from the effects of deflections in the hullstructure.
 4. The system of claim 1, further comprising: a plurality ofenergy absorption and transfer structures positioned between the hullstructure and the concrete foundation, the plurality of energyabsorption and transfer structures configured to: absorb at least aportion of the forces or kinetic energy transferred from the turbinestructure, and transfer at least another portion of the forces orkinetic energy from the turbine structure to the hull structure.
 5. Thesystem of claim 4, wherein at least some of the plurality of energyabsorption and transfer structures are springs.
 6. The system of claim1, wherein the hull structure further comprises: structural bracketsconfigured to transfer forces or kinetic energy received, at the hullstructure from the turbine structure, to an outside of the hull and anadded mass of the water surrounding the hull structure.
 7. The system ofclaim 1, wherein the hull structure further comprises: a plurality ofballasts configured to adjust a buoyancy of the hull structure.
 8. Thesystem of claim 1, further comprising: a plurality of adjustmentstructures positioned between the hull structure and the concretefoundation, the adjustment structures configured to adjust the degree ofstiffness between the hull structure and the concrete foundation.
 9. Thesystem of claim 8, wherein at least some of the plurality of adjustmentstructures are hydraulic jacks.
 10. The system of claim 9, wherein atleast some of the plurality of adjustment structures are configured toabsorb forces or kinetic energy from the turbine structure.
 11. A systemcomprising: a hull structure configured to float in water; a foundationmounted on top of the hull structure, the foundation configured tosupport a turbine structure; and at least one of: a plurality ofadjustment structures positioned between the hull structure and thefoundation, the adjustment structures configured to adjust the degree ofstiffness between the hull structure and the foundation, and a pluralityof energy absorption and transfer structures positioned between the hullstructure and the foundation, the plurality of energy absorption andtransfer structures configured to do at least one of: isolate thefoundation from the effects of deflections in the hull structure, andabsorb at least a portion of the forces or kinetic energy transferredfrom the turbine structure, and transfer at least another portion of theforces or kinetic energy from the turbine structure to the hullstructure.
 12. The system of claim 11, comprising both of the pluralityof adjustment structures and the plurality of energy absorption andtransfer structures.
 13. The system of claim 12, wherein at least someof the plurality of energy absorption and transfer structures aresprings.
 14. The system of claim 12, wherein at least some of theplurality of adjustment structures are hydraulic jacks.
 15. The systemof claim 12, wherein at least some of the plurality of adjustmentstructures are configured to absorb forces or kinetic energy from theturbine structure.
 16. The system of claim 11, wherein the hullstructure further comprises: structural brackets configured to transferforces or kinetic energy received, at the hull structure from theturbine structure, to an outside of the hull and an added mass of thewater surrounding the hull structure.
 17. The system of claim 11,wherein the hull structure further comprises: a plurality of ballastsconfigured to adjust a buoyancy of the hull structure.
 18. A systemcomprising: a hull structure configured to float in water; a foundationmounted on top of the hull structure, the foundation configured tosupport a turbine structure; a plurality of adjustment structurespositioned between the hull structure and the foundation, the adjustmentstructures configured to: adjust the degree of stiffness between thehull structure and the foundation, and absorb forces or kinetic energyfrom the turbine structure; and a plurality of energy absorption andtransfer structures positioned between the hull structure and thefoundation, the plurality of energy absorption and transfer structuresconfigured to: isolate the foundation from the effects of deflections inthe hull structure, and absorb at least a portion of the forces orkinetic energy transferred from the turbine structure, and transfer atleast another portion of the forces or kinetic energy from the turbinestructure to the hull structure.
 19. The system of claim 18, wherein thefoundation is a composite made of concrete.
 20. The system of claim 18,wherein the hull structure further comprises: a plurality of ballastsconfigured to adjust a buoyancy of the hull structure.